Hey not sure if you bought another betta. But i would really disinfect the tank though
Ok someone on this forum wrote this, and i agree with that but i am too too careful and paranoid so i always like to be safe then sorry. So that what someone wrote:

"A lot of aquatic diseases are sensitive to dryness so simply leaving the tank dry for a couple weeks will help sanitize things.

For everyday, basic cleaning, I use a combo of vinegar and boiling water. I don't have a set ratio but I would recommend at least 1:10 vinegar to water.

If you are sanitizing after a disease outbreak, I would do a bleach soak+through drying. Use a 1:10 bleach water ratio.''

It's easy to disinfect the tank , but it pain to disinfect the gravel and decorations, or plants. So that is why i always recommending to quarantine the fish, even if you don't put it in the tank with other fish. I quarantine new fish even if i see that betta is healthy , because you never know .....

Now abut bleach disinfection, i am going to give you the link. Check my post about the disinfection , but read it thoroughly before you do it. And remember to wash it a lot and let it sit to dry out to neutralize the bleach.

I did disinfect a few times with a bleach a while ago but i never disinfect the decor or gravel. When you read the post you will see the gravel recommendations. You CAN NOT disinfect the porous gravel with a bleach.

I did boiled the stones before long time ago and then dry them in the son for a few days , but some people saying that stones can burst while you boiling them, so be careful if you decide to do it. And if you will boil them let them to cool down completely before you spill the water out.

just saw your other thread i was wondering how old he is. Hmm i am curious why he got sick.I think you was doing pretty good job with water changes.
Do you vacuum the gravel?
Also make sure you rinse/swish the filter media weekly in the tank water
Also some one was saying i think in your other thread that you can not use Epsom salt on the betta. I just want to say that it absolutely safe to use Epsom salt for bettas when needed , just make sure it the right dose.